Homeless: Now, this is from a picture I saw of the reservoir on ebay, it has four holes, two at the top and two on the bottom. Frankly, it seems overkill to me given that the engine coolant reservoir is held in by a strap!
And, I've reached under the windshield washer reservoir and felt nuts on bolts at the corners. Getting a wrench or socket on them: Not easy at first glance.
94-Newmar: I have plenty of 3S lipos (12.6 volts dc) I can use for the exact purpose of external power. Thanks for the suggestion!
Actually, it might be best if the pump didn't run because then it's likely something else like a bad switch or wiring. One of my friends who works at Allied Recreation Group suggested it might be controlled via logic, but I kind of doubt it because of age. If it is wiring, it seems to be little trouble to wire a push button directly.